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Asymmetric Hydroboration Reactions

Rhodium-catalysed asymmetric cyclization/hydroboration followed either by Pd-catalysed arylation or by oxidation was applied to the synthesis of a number of chiral, non-racemic carbocycles and heterocycles. Thus, reaction of enyne (28) with catecholborane, catalysed by a 1 1 mixture of [Rh(COD)2]+ Sbly,- and (S)-BINAP (5 mol%), followed by Pd-catalysed arylation with /7-IC6H4CF3, afforded benzyli-denecyclopentane (29) in 65% yield with 88% ee.46... [Pg.325]

Hydroboration-reduction of enones.2 Hydride reduction of a carbonyl group can be used to induce asymmetric intramolecular hydroboration of a double bond via a cyclic transition state. Thus reaction of the enone 1 with thexylborane (1 equiv.) followed by oxidation provides the 1,5-diol 2 with high 1,4-syn selectivity. A similar reaction with the homologous enone provides a 1,6-diol with modest 1.5-syn selectivity (syn anti = 6.6 1). [Pg.327]

There are two common ways to accomplish an asymmetric reaction. Either a second chiral center is created in a molecule under the influence of an existing chiral center in that molecule or a chiral reagent acts on a prochiral substrate to create a new chiral center. The conversion of chiral a-keto esters to di-, astereomeric a-hydroxy esters is an example of the first type of asymmetric reaction, and the asymmetric hydroboration of alkenes with chiral boranes is an example of the second type (Fig. 1). [Pg.82]

Asymmetric hydroboration and conceptually similar reactions involving chiral reagents have been used with great success. Their principal shortcoming is that stoichiometric quantities of chiral compounds must be invested and only rarely can these compounds be recycled. An asymmetric reaction involving a catalyst that is chiral would be a superior way to accomplish an asymmetric synthesis since, with only a small amount of chiral material, large quantities of optically active product could, in principle, be obtained. [Pg.83]

Asymmetric reaction is one of the most exciting features of catalyzed hydroboration since optically active phosphine ligands are the chiral auxiliaries most extensively studied for metal-catalyzed reactions (Scheme 13).134 The chiral ligands used for asymmetric hydroboration of alkenes include BINAP,136 1 03-106,167-170 QUINAP,171-173 107-109,172,174-176 and BDPP.177,178... [Pg.155]

Hydroboration and asymmetric reductions proceed with high stereoselectivity because of the small atom radius and the large electronegativity of boron [22]. Hence, organoboron compounds are used for asymmetric reactions, especially for asymmetric reduction [9,22,36]. Both the yield and the optical yield of asymmetric reactions with organoboron compounds are high, and the isolation of chiral products is easy. Therefore, these reactions are exploited for syntheses of terpenes [22]. [Pg.100]

Hydroboration of Ketones with Boranes from ot-Pinenes and the Non-linear Effect in Asymmetric Reactions... [Pg.144]

One of the newer and more fmitful developments in this area is asymmetric hydroboration giving chiral organoboranes, which can be transformed into chiral carbon compounds of high optical purity. Other new directions focus on catalytic hydroboration, asymmetric aHylboration, cross-coupling reactions, and appHcations in biomedical research. This article gives an account of the most important aspects of the hydroboration reaction and transformations of its products. For more detail, monographs and reviews are available (1—13). [Pg.308]

There have been several reviews of asymmetric synthesis via chiral organoboranes (6,8,378,382,467—472). Asymmetric induction in the hydroboration reaction may result from the chiraHty present in the olefin (asymmetric substrate), in the reagent (asymmetric hydroboration), or in the catalyst (catalytic asymmetric hydroboration). [Pg.321]

High levels of asymmetric induction have been achieved in the hydroboration of 1,3-, 1,4-, and 1,5-dienes with thexylborane (482,483,489,490). The first chiral center is formed by an intermolecular reaction. In the second step, the organoborane intermediate undergoes an intramolecular hydroboration, creating the second chiral center with high diastereoselectivity. [Pg.322]

EinaHy, kinetic resolution of racemic olefins and aHenes can be achieved by hydroboration. The reaction of an olefin or aHene racemate with a deficient amount of an asymmetric hydroborating agent results in the preferential conversion of the more reactive enantiomer into the organoborane. The remaining unreacted substrate is enriched in the less reactive enantiomer. Optical purities in the range of 1—65% have been reported (471). [Pg.323]

Dilongifolyl borane [77882-24-7] M 422.6, m 169-172 . Wash with dry Et20 and dry in a vacuum under N2. It has m 160-161 in a sealed evacuated capillary. It is sparingly soluble in pentane, tetrahydrofuran, carbon tetrachloride, dichloromethane, and chloroform but the suspended material is capable of causing asymmetric hydroboration. Disappearance of solid indicates that the reaction has proceeded. [J Org Chem 46 2988 1981.]... [Pg.419]

The synthesis of 10 features the SN2 displacement of the allylic acetate with migration of R2 from the ate complex6. Precursors 9 are prepared by the hydroboration of 3-acetoxy-l-alkynes that are available with very high enantiomeric purity via the asymmetric reduction of the corresponding l-alkyn-3-ones, and a substantial degree of asymmetric induction occurs in the conversion of 9 to 10. Best results, based on the enantioselectivity of reactions of 10 with aldehydes, are obtained when R2 is a bulky group such as isopinocampheyl (79 85 % ee)6. The yields of reactions of 10 with aldehydes are 62-76%. [Pg.314]

In addition to the enhanced rate of hydroalumination reactions in the presence of metal catalysts, tuning of the metal catalyst by the choice of appropriate ligands offers the possibility to influence the regio- and stereochemical outcome of the overall reaction. In particular, the use of chiral ligands has the potential to control the absolute stereochemistry of newly formed stereogenic centers. While asymmetric versions of other hydrometaUation reactions, in particular hydroboration and hydrosi-lylation, are already weU established in organic synthesis, the scope and synthetic utiHty of enantioselective hydroalumination reactions are only just emerging [72]. [Pg.63]

This route can be used to prepare enantiomerically enriched organozinc reagents by asymmetric hydroboration (see Section 4.5.3), followed by exchange with diisopropyl-zinc. Trisubstituted cycloalkenes such as 2-methyl or 2-phenylcyclohexene give an enantiomeric purity greater than 95%. The exchange reaction takes place with retention of configuration.137... [Pg.652]

Formal hydration of the double bond appeared by the hydroboration-oxidation sequence. Desymmetrization reactions with catalytic asymmetric hydroboration are not restricted to norbornene or nonfunctionalized substrates and can be successfully applied to meso bicyclic hydrazines. In the case of 157, hydroxy derivative 158 is formed with only moderate enantioselectivity both using Rh or Ir precatalysts. Interestingly, a reversal of enantioselectivity is observed for the catalytic desymmetrization reaction by exchanging these two transition metals. Rh-catalyzed hydroboration involves a metal-H insertion, and a boryl migration is involved when using an Ir precatalyst (Equation 17) <2002JA12098, 2002JOC3522>. [Pg.392]

In 1993, Hayashi and co-workers reported a catalytic asymmetric synthesis of alle-nylboranes 256 by palladium-catalyzed hydroboration of conjugated enynes 253 (Scheme 4.66) [105]. Reaction of but-l-en-3-ynes 253 with catecholborane 254 in the presence of a catalyst, prepared from Pd2(dba)3 CHC13 (1 mol%) and a chiral mono-dentate phosphine ligand (S)-MeO-MOP 255 (1 mol%), gave an allenylborane 256. The ee of 256 was determined by the reaction with benzaldehyde affording the corresponding optically active homopropargyl alcohols 257 with up to 61% ee (syn anti= 1 1—3 1). [Pg.172]

Boronic esters have been used in a wide range of transformations. These useful reagents have been transformed into numerous functional groups and are essential reagents for several C-C bond-forming reactions. Transition metal-catalyzed hydroboration of olefins often leads to mixtures of branched and linear products. Several groups have reported asymmetric reductions of vinyl boronic esters [50-52] with chiral rhodium P,P complexes however, the first iridium-catalyzed reduction was reported by Paptchikhine et al (Scheme 10) [53]. [Pg.49]


See other pages where Asymmetric Hydroboration Reactions is mentioned: [Pg.101]    [Pg.22]    [Pg.139]    [Pg.71]    [Pg.121]    [Pg.105]    [Pg.280]    [Pg.105]    [Pg.22]    [Pg.139]    [Pg.122]    [Pg.126]    [Pg.324]    [Pg.247]    [Pg.23]    [Pg.191]    [Pg.192]    [Pg.192]    [Pg.195]    [Pg.196]    [Pg.207]    [Pg.9]    [Pg.19]    [Pg.22]    [Pg.1173]    [Pg.845]    [Pg.253]    [Pg.938]    [Pg.447]    [Pg.179]    [Pg.180]   
See also in sourсe #XX -- [ Pg.342 ]

See also in sourсe #XX -- [ Pg.156 , Pg.157 ]

See also in sourсe #XX -- [ Pg.174 ]




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